Miniaturization of modern components, such as the integrated circuits (IC) and other electronic components from which they are assembled, is helpful in reducing the size of mobile devices. Further size reduction may be achieved by efficient electrical and mechanical (e.g., electromechanical) packaging of the electronic components from which the mobile devices are assembled. Modern electronic packaging techniques, such as stacked-die packaging, package-in-packages (PiP) and stacked-packaging, thus increase the density of components in mobile device assembly.
To conserve planar space on a mutual motherboard, discrete interacting IC components of a mobile device such as logic and memory may be stacked, e.g., installed vertically in relation to each other, using a package-on-package (PoP) technique. In a typical PoP mixed logic-memory stack, a central processing unit (CPU) or other processor package may be electro-mechanically mounted (electrically coupled and mechanically fastened) to a printed circuit board (PCB) using a ball-grid array (BGA) or other conductors. The memory package may be disposed over the CPU package.
The memory package is electro-mechanically mounted to the CPU package using a BGA. The CPU package and the memory package each comprise an electrically insulating laminate substrate, which is disposed over an upper surface of their respective BGAs. A logic die is disposed within the substrate of the CPU package and a memory die is disposed within the memory package substrate. As memory uses fewer input/output (I/O) resources than logic, the connection pitch (e.g., density; solder ball count) of electrical interconnects for a logic package typically exceeds the connection pitch of electrical interconnects for a memory package.
In another PoP, a CPU package is disposed over a memory package using a single laminate substrate base on which two BGAs are disposed. A BGA with a fine ball pitch may be disposed over a first substantially rectangular area, which occupies a substantially central area of the base. A second BGA with coarse pitch, in relation to the fine ball pitch of the first BGA, is disposed over a pad. The pad comprises a recess disposed within a second substantially rectangular area of the base, which essentially surrounds an outer boundary of the first central area and BGA.
PoPs are typically thin, which allow their use in small (e.g., thin) mobile devices. The discrete CPU and random access memory (RAM) component packages from which the PoPs are assembled, are thus also quite thin. However, it may be difficult to maintain the flatness and/or prevent flexing and damage to such thin PoPs and their component packages during and following assembly. One stiffening approach to address this issue uses plastic, polymeric or ceramic stiffener rings. However, stiffeners are useless or inapplicable in some PoPs and may lack sufficient strength and/or stiffness to control warping or flexing, or to prevent related damage to the PoP assembly or its components during fabrication, installation or operation. Where used, stiffener rings also demand space, typically including an exclusionary zone adjacent or proximate to its periphery, which reduces available operational “real estate” on the PoP.
Further, PoP assemblies may have an electrically insulating polymeric matrix between the upper RAM component and the lower CPU component for support, and to encapsulate and insulate the first (e.g., fine pitch, central) electrical interconnections. Before the polymeric under-fill material fully sets during assembly of a PoP, it may tend to flow. An over flow of the under-fill polymer insulation however may contaminate the outer pad. Contamination of the outer pad can reduce the electrical integrity of the second BGA disposed therein.
One approach to address this issue is to add an insulating material as a dam, to block the overflow of under-fill material from encroaching on the dam area. However, this requires several extra process steps, which complicate package fabrication. For example, a first additional process step is needed to add the dam material and, after the under-fill material sets, a second process step is needed to remove the dam material. Unfortunately, the polymeric dam has an insufficient modulus of stiffness to function as a stiffener. Moreover, the second additional removal process step is also needed to address issues related to one or more electrical, chemical, thermal, mechanical or other properties of the dam material.
Approaches described in this Background section could, but have not necessarily been conceived or pursued previously. Unless otherwise indicated, neither approaches described in this section, nor issues identified in relation thereto, are to be assumed as recognized in any prior art merely by the discussion thereof within this section.